Hydronic System Types

• Hydronic systems use water through pipes to heat or cool a space. In a hydronic system, water flows in an open loop or a closed loop. In an open loop, excess water is exposed to outside air at its terminal, typically a cooling tower. In a closed loop, excess water is not exposed to the outside air. Cooling systems are typically open loops and heating systems are typically closed loops.
  
  

Expansion Tank Purpose

• In a closed loop hydronic system, water is heated in the boiler and runs through the pipes. The heated water expands and adds pressure to the system. The expansion tank takes in the extra water, relieving the pressure. It is essential to have an expansion tank in a closed loop hydronic system.
  
  

Steel Expansion Tank

• The steel expansion tank is the older version of the cushion tank. In this tank, the pressurized air and the excess water exist together and are not separated by a diaphragm. It is necessary to maintain air pressure in the expansion tank because the pressurized air absorbs the water pressure. The air compresses when the hot water reaches it and the pressure in the tank is maintained.
  
  

Bladder Expansion Tank

• The bladder, or diaphragm, expansion tank separates the pressurized air and the excess water with a diaphragm. This type of tank is typically found attached directly to the boiler piping and not hanging from the ceiling as steel expansion tanks typically are. The size of the bladder expansion tank is directly related to the size of the boiler. Using the correct size expansion tank for a boiler is essential to preventing pressure problems in the system.